An item that is frequently sold to the Clueless is a portal detector. Now, the idea of a portal detector is great. Especially if it could tell you where the respective portals lead to. But do they work? Of course, most of them don't work, or lead you to very obvious gateways. Some even lead you to specific portals which end up in places where you are enslaved at the very least. The few of them that do work, or at least detect a certain kind of portal, are rarely sold market places - and it they are, they are bound to cost a stupid price. Some of the dabus are rumoured to have portal detectors so they can monitor and maintain certain entrances into the city.
Most portal detectors look like an ordinary gem (if there is such a thing) or are figurines crafted from precious stones. These figurines often symbolise the type of portal they are attuned to. The gems and figurines vibrate or flicker when they come near the type of portal they are meant to detect. In rare cases, the portal detectors are made from other materials such as metal, wood or bone.
Here are some more unusual sample detectors (these are not portal keys, though some might be):
Obsidian Salamander statue - Portal detector for gates to Gehenna
Bone or dung statue (or animal skull) - Portal detector for Beastlands
Flickering Ruby - Portal detector for Elemental Plane of Fire (gets hot or bursts into flames near a suitable portal)
Clay, stone or coal statue - Portal detector for Elemental Plane of Earth
Glass prism - Portal detector for Quasi-Elemental Plane of Radiance
Piece of lava - Portal detector for Para-Elemental Plane of Magma
Mechanical detector (resembling a pocket watch) - Portal detector for Mechanus